Since 1.6, iptables supports the MASQUERADE target options:

MASQUERADE target options:
 --to-ports <port>[-<port>]
				Port (range) to map to.
 --random
				Randomize source port.
 --random-fully
				Fully randomize source port.

When building iptables-nft, the MASQUERADE target options are included in the iptables-nft binary, however, they are not in the ip6tables-nft:

$> sudo ./output/target/usr/sbin/iptables-nft -A POSTROUTING -s 10.244.0.0/16 ! -d 224.0.0.0/4 -j MASQUERADE --help | grep random
 --random
 --random-fully
				Fully randomize source port.
$> sudo ./output/target/usr/sbin/ip6tables-nft -A POSTROUTING -s 10.244.0.0/16 ! -d 224.0.0.0/4 -j MASQUERADE --help | grep random
$>

Those targets come from the files libipt_MASQUERADE.c and libip6t_MASQUERADE.c in https://git.netfilter.org/iptables/tree/extensions but for some reason the ip6 one does not get included in the binary